The present invention relates to a photosensitive thermosetting resin composition that has a long shelf life and has excellent normal temperature storage stability, has good solder heat resistance after curing, and according to which high adhesive strength can be maintained, and to a photosensitive cover lay and a flexible printed wiring board using the composition.
Various art relating to photosensitive cover lays (photosensitive dry films) and to photosensitive thermosetting resin compositions for forming such photosensitive cover lays and so on has been proposed hitherto.
For example, in Japanese Patent Application Laid-open No. 2005-62450, there is disclosed a resist ink photosensitive thermosetting resin composition for which the cured material has excellent folding resistance (flexibility) and so on. Specifically, this is a photosensitive thermosetting resin composition containing (A) a specified carboxyl-modified epoxy (meth)acrylate resin, (B) a biphenyl type epoxy resin, (C) a photopolymerization initiator, and (D) a diluent, wherein the carboxyl-modified epoxy (meth)acrylate resin (A) has an acid value within a specified range.
Moreover, in Japanese Patent Application Laid-open No. H10-20493, there is disclosed a solder-resist ink photosensitive thermosetting resin composition. Specifically, this is a photopolymerizable thermosetting resin composition comprising (A) an active energy ray-curable resin (a), (B) a diluent, (C) a photopolymerization initiator, (D) a cured adhesion bestowing agent, and (E) an epoxy group-containing compound, wherein the active energy ray-curable resin (a) is selected from specified unsaturated resins.
Moreover, in Japanese Patent Application Laid-open No. 2003-241369, there is disclosed a dry film resist the object of which is to improve peelability between a protective film and a photosensitive layer. Furthermore, in Japanese Patent Application Laid-open No. 2003-241369, description is given of photosensitive prepolymers, cracking improving agents, photopolymerization initiators (including sensitizers), fire retardants, fillers, and so on that can be used in the dry film resist.
Moreover, in Japanese Patent Application Laid-open No. 2003-195486, there is disclosed a photosensitive composition the object of which is to improve performance relating to the formation of a photosensitive coating film such as photosensitivity, developability, and shelf life. Specifically, this is a photosensitive composition containing (A) a compound having an ethylenic unsaturated group, (B) an epoxy resin, (C) a photopolymerization initiator, and (D) an interlayer compound in which a specified thermal polymerization catalyst is inserted into an inorganic layer compound.
For a photosensitive cover lay formed using such a conventional photosensitive thermosetting resin composition, it has been difficult to obtain a photosensitive thermosetting resin composition that has a long shelf life and has excellent normal temperature storage stability, has good solder heat resistance after curing, and according to which high adhesive strength can be maintained. Moreover, even in the case of a photosensitive thermosetting resin composition satisfying these characteristics, the flexibility after curing has sometimes been insufficient, and hence handling has been difficult.